Acta Crystallographica Section B

Structural Science

Volume 58, Part 6 (December 2002)

research papers

Acta Cryst. (2002). B58, 1018-1024 [ doi:10.1107/S0108768102018797 ]

Pressure-induced polymorphism in phenol

D. R. Allan, S. J. Clark, A. Dawson, P. A. McGregor and S. Parsons

Abstract: The high-pressure crystal structure of phenol (C₆H₅OH), including the positions of the H atoms, has been determined using a combination of single-crystal X-ray diffraction techniques and ab initio density-functional calculations. It is found that at a pressure of 0.16 GPa, which is just sufficient to cause crystallization of a sample held at a temperature just above its ambient-pressure melting point (313 K), a previously unobserved monoclinic structure with P2₁ symmetry is formed. The structure is characterized by the formation of hydrogen-bonded molecular chains, and the molecules within each chain adopt a coplanar arrangement so that they are ordered in an alternating 1-1-1 sequence. Although the crystal structure of the ambient-pressure P112₁ phase is also characterized by the formation of molecular chains, the molecules adopt an approximate threefold arrangement. A series of ab initio calculations indicates that the rearrangement of the molecules from helical to coplanar results in an energy difference of only 0.162 eV molecule^-1 (15.6 kJ mole^-1) at 0.16 GPa. The calculations also indicate that there is a slight increase in the dipole moment of the molecules, but, as the high-pressure phase has longer hydrogen-bond distances, it is found that, on average, the hydrogen bonds in the ambient-pressure phase are stronger.

Keywords: phenol; polymorphism; single-crystal X-ray diffraction; density-functional calculations.


	PDF file (23.8 kbytes) Refined fractional coordinates for the high-pressure structures

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